In psychology, a dual process theory provides an account of how a phenomenon can occur in two different ways, or as a result of two different processes. Often, the two processes consist of an implicit (automatic), unconscious process and an explicit (controlled), conscious process. Verbalized explicit processes or attitudes and actions may change with persuasion or education; though implicit process or attitudes usually take a long amount of time to change with the forming of new habits. Dual process theories can be found in social, personality, cognitive, and clinical psychology.

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The foundations of dual process theory likely comes from William James. He believed that there were two different kinds of thinking: associative and true reasoning. James theorized that empirical thought was used for things like art and design work. For James, images and thoughts would come to mind of past experiences, providing ideas of comparison or abstractions. He claimed that associative knowledge was only from past experiences describing it as “only reproductive”. James believed that true reasoning was useful for “unprecedented situations” in which using reasoning to overcome obstacles such as navigation could be overcome with reasoning power of being able to use a map.

Steven Sloman produced another interpretation on dual processing. He believed that associative reasoning takes stimuli and divides it into logical clusters of information based on statistical regularity. He proposed that how you associate is directly proportional to the similarity of past experiences, relying on temporal and similarity relations to determine reasoning rather than an underlying mechanical structure. The other reasoning process in Sloman's opinion was of the Rule based system. The system functioned on logical structure and variables based upon rule systems to come to conclusions different from that of the associative system. He also believed that the Rule based system had control over the associative system, though it could only suppress it.[1]

Daniel Kahneman provided further interpretation by differentiating the two styles of processing more, calling them intuition and reasoning. Intuition (or system 1), similar to associative reasoning, was determined to be fast and automatic, usually with strong emotional bonds included in the reasoning process. Kahneman said that this kind of reasoning was based on formed habits and very difficult to change or manipulate. Reasoning (or system 2) was slower and much more volatile, being subject to conscious judgments and attitudes.[2]

Ron Sun proposed a dual process model of learning (both implicit learning and explicit learning). The model (named CLARION) re-interpreted voluminous behavioral data in psychological studies of implicit learning and skill acquisition in general. The resulting theory is two-level and interactive, based on the idea of the interaction of one-shot explicit rule learning (i.e., explicit learning) and gradual implicit tuning through reinforcement (i.e. implicit learning), and it accounts for many previously unexplained cognitive data and phenomena based on the interaction of implicit and explicit learning.[4]

Using a somewhat different approach, Allan Paivio has developed a dual-coding theory of information processing. According to this model, cognition involves the coordinated activity of two independent, but connected systems, a nonverbal system and a verbal system that is specialized to deal with language. The nonverbal system is hypothesized to have developed earlier in evolution. Both systems rely on different areas of the brain. Paivio has reported evidence that nonverbal, visual images are processed more efficiently and are approximately twice as memorable. Additionally, the verbal and nonverbal systems are additive, so one can improve memory by using both types of information during learning.[5]

The dual-process accounts of reasoning posits that there are two systems or minds in one brain. The theory of two distinct kinds of reasoning has been around for as long as documentations about theories of reasoning go. The current theory is that there are two distinctively separate cognitive systems underlying thinking and reasoning and that these different systems were developed through evolution.[6] These systems are often referred to as being either implicit and explicit, however some theorists prefer to emphasize the functional differences between the two systems and not the consciousness factor and thus refer to the systems simply as System 1 and System 2. The broad terms System 1 and System 2 were coined by Stanovich and West[7] and will be used throughout this article.

System 1 is automatic and unconscious. Some other terms used for the system are the implicit system, the experiential system, the associative system,[6] and the heuristic system.[8] It contains the universal cognition that is shared between humans and animals and is thus considered evolutionarily old.[6] This is not a single system, it is a set of subsystems that do operate with some autonomy.[7] The system itself is domain-general, however the autonomy of the subsystems reflects domain-specific learning.[6] This system controls instinctive behaviours that are innately programmed.[9] It tends to solve problems by relying on prior knowledge and belief[8] and produces rapid, parallel and automatic processes where only the final product is conscious. Some overall properties associated with System 1 are that it is associative, holistic, automatic, relatively undemanding of cognitive capacity, and fast.[6]

System 2 is evolutionarily recent and specific to humans. It is also known as the explicit system, the rule-based system, the rational system,[6] or the analytic system.[8] It performs the more slow and sequential thinking. It is domain-general, performed in the central working memory system. Because of this, it has a limited capacity and is slower than System 1 which correlates it with general intelligence. This system permits the abstract hypothetical thinking that is not permitted in System 1 and is distinct to humans.[6] It is known as the rational system because it reasons according to logical standards.[8] Some overall properties associated with System 2 are that it is rule-based, analytic, controlled, demanding of cognitive capacity, and slow.[6]

A belief bias is found when participants in a study are asked to assess the logical validity of arguments where the conclusions are either believable or unbelievable. Deductive reasoning is the process of drawing these valid conclusions from a given set of premises. It is one of the most important cognitive processes.[8] The results are that there is both a logical process (System 2) and a belief-based process (System 1) that are in competition with each other in evaluating the argument. The ability to solve these logical arguments is correlated with measures of general cognitive intelligence and their abilities to do this decline with age. It is very hard to suppress the influence of System 1, the prior knowledge about the believability of a conclusion, and focus solely on the logic of the premises to the conclusion.[6]
Studies on belief-bias effect were first designed by Jonathan Evans to create a conflict between logical reasoning and prior knowledge about the truth of conclusions. To do this they presented syllogisms for evaluation by the participants which fall into one of four categories: valid argument with believable conclusions, valid argument with unbelievable conclusions, invalid argument with believable conclusion, and invalid argument and unbelievable conclusion.[6] The trials with conclusions that are consistent with beliefs about the world are called congruent, and those that are not are called incongruent.[8] Participants are told to only agree with conclusions that follow the premises given. The results show that the participants decisions on the believability of the conclusions are consistently influenced by prior knowledge. An example is that when there is an invalid argument but the conclusion is believable, a person may choose to accept the conclusion even if it does not follow the premises, this is a belief-bias. It is the interference of prior knowledge of System 1 onto the logic of System 2.[6]

De Neys[11] conducted a study that manipulated working memory capacity while answering syllogistic problems. This was done by burdening executive processes with secondary tasks. Results showed that when System 1 triggered the correct response, the distractor task had no effect on the production of a correct answer which supports the fact that System 1 is automatic and works independently of working memory, but when belief-bias was present (System 1 belief-based response was different from the logically correct System 2 response) the participants performance was impeded by the decreased availability of working memory. This falls in accordance with the knowledge about System 1 and System 2 of the dual-process accounts of reasoning because System 1 was shown to work independent of working memory, and System 2 was impeded due to a lack of working memory space so System 1 took over which resulted in a belief-bias.[11]

Vinod Goel produced neuropsychological evidence for dual-process accounts of reasoning using fMRI[12] studies. They provided evidence that anatomically distinct parts of the brain were responsible for the two different kinds of reasoning. They proved that content-based reasoning caused left temporal hemisphere activation whereas abstract formal problem reasoning activated the parietal system. They concluded that different kinds of reasoning, depending on the semantic content, activated one of two different systems in the brain.[12]
A similar study using fMRI incorporated the belief-bias test into the test.[13] They found that different mental processes were competing for control of the response to the problems given in the belief-bias test. The prefrontal cortex was critical in detecting and resolving conflict which are characteristic of System 2 and an area already typically associated with that same system. The ventral medial prefonrtal cortex, known to be associated with the more intuitive or heuristic responses of System 1 was the area in competition with the prefrontal cortex.[13]

Tsujii and Watanabe[8] did a follow-up study to Goel and Dolan's[13] fMRI experiment. They examined the neural correlates on the inferior frontal cortex (IFC) activity in belief-bias reasoning using near-infrared spectroscopy (NIRS). Subjects performed a syllogistic reasoning task, using congruent and incongruent syllogisms while attending to an attention-demanding secondary task. Their interest was in how the secondary-tasks changed the activity of the IFC during congruent and incongruent reasoning processes. The results showed that the participants performed better in the congruent test than the incongruent test (evidence for belief-bias), the high demand secondary test impaired incongruent reasoning results but not congruent reasoning. NIRS results showed that the right IFC was activated more during incongruent trials. Participants with enhanced right IFC activity performed better on the incongruent reasoning than those with decreased right IFC activity. This study provided some evidence to enhance the fMRI results that the right IFC, specifically, is critical in resolving conflicting reasoning but is attention-demanding, its effectiveness decreases with loss of attention where the automatic heuristic System 1 takes over which results in a belief-biases.[8]

Matching bias is a non-logical heuristic.[14] The matching bias is described as a tendency to use lexical content matching of the statement about which one is reasoning to be seen as relevant information and do the opposite as well, ignore relevant information that doesn't match. It mostly affects problems with abstract content. It doesn't involve prior knowledge and beliefs but it is still seen as a System 1 heuristic that competes with the logical System 2.[14]

The Wason selection task provides evidence for the matching bias.[6] The test is designed as a measure of a person's logical thinking ability.[15] Performance on the Wason Selection Task is sensitive to the content and context with which it is presented. If you introduce a negative component into the conditional statement of the Wason Selection Task, e.g. 'If there is an A one side of the card then there is not a 3 on the other side', there is a strong tendency to choose cards that match the items in the negative condition to test, regardless of their logical status. Changing the test to be a test of following rules rather than truth and falsity is another condition where the participants will ignore the logic because they will simply follow the rule, e.g. changing the test to be a test of a police officer looking for underaged drinkers.[14] The original task is more difficult because it requires explicit and abstract logical thought from System 2, and the police officer test is cued by relevant prior knowledge from System 1.[6]
Studies have shown that you can train people to inhibit matching bias which provides neuropsychological evidence for the dual-process theory of reasoning.[6] When you compare trials before and after the training there is evidence for a forward shift in activated brain area. Pre-test results showed activation in locations along the ventral pathway and post-test results showed activation around the ventro-medial prefrontal cortex and anterior cingulate.[16]

Dual-process theorists claim that System 2, a general purpose reasoning system, evolved late and worked alongside the older autonomous sub-systems of System 1.[17] The success of Homo Sapiens Sapiens lends evidence to their higher cognitive abilities above other hominids. Mithen theorizes that the increase in cognitive ability occurred 50,000 years ago when representational art, imagery, and the design of tools and artefacts are first documented. She hypothesizes that this change was due to the adaptation of System 2.[17]
Most evolutionary psychologists do not agree with dual-process theorists. They claim that the mind is modular, and domain-specific, thus they disagree with the theory of the general reasoning ability of System 2. They have difficulty agreeing that there are two distinct ways of reasoning and that one is evolutionarily old, and the other is new.[6] To ease this discomfort, the theory is that once System 2 evolved it became a 'long leash' system without much genetic control which allowed humans to pursue their individual goals.[7]

The dual-process account of reasoning is an old theory, as noted above. But according to Evans[18] it has adapted itself from the old, logicist paradigm, to the new theories that apply to other kinds of reasoning as well. And the theory seems more influential now than in the past which is questionable. Evans outlined 5 "fallacies":

All dual-process theories are essentially the same. There is a tendency to assume all theories that propose two modes or styles of thinking are related and so they end up all lumped under the umbrellaa term of "dual-process theories".

There are just two systems underlying System 1 and System 2 processing. There are clearly more than just two cognitive systems underlying people's performance on dual-processing tasks. Hence the change to theorizing that processing is done in two minds that have different evolutionary histories and that each have multiple sub-systems.

System 1 processes are responsible for cognitive biases; System 2 processes are responsible for normatively correct responding. Both System 1 and System 2 processing can lead to normative answers and both can involve cognitive biases.

System 1 processing is contextualised while System 2 processing is abstract.[18] Recent research has found that beliefs and context can influence System 2 processing as well as System 1.[19]

Fast processing indicates the use of System 1 rather than System 2 processes. Just because a processing is fast does not mean it is done by System 1. Experience and different heuristics can influence System 2 processing to go faster.[18]

Another argument against dual-process accounts for reasoning was outlined by Osman is that the proposed dichotomy of System 1 and System 2 do not adequately accommodate the range of processes accomplished.[20] Moshman proposed that there should be four possible types of processing as opposed to two. They would be implicit heuristic processing, implicit rule-based processing, explicit heuristic processing, and explicit rule-based processing.[21]
In response to the question as to whether there are dichotomous processing types, many proposed a single-system framework instead that incorporates a continuum between implicit and explicit processes.[20]

The dynamic graded continuum (DGC), originally proposed by Cleermans and Jimenez is an alternative single system framework to the dual-process account of reasoning. It has not been accepted as better than the dual-process theory, it is instead usually used as a comparison with which one can evaluate the dual-process model. The DGC proposes that differences in representation generate variation in forms of reasoning without assuming a multiple system framework. It describes how graded properties of the representations that are generated while reasoning result in the different types of reasoning. It separates terms like implicit and automatic processing where the dual-process model uses the terms interchangeably to refer to the whole of System 1. Instead the DGC uses a continuum of reasoning that moves from implicit, to explicit, to automatic.[20]